Separate classes of Mouse Mammary Tumor Virus (MMTV) exist which differ in their mode of transmission to progeny and in the incidence of mammary tumors they produce. The objective of this study is to relate these biological differences with the differences in the virus specific nucleic acids present in normal and mammary tumor tissues and with the relative arrangement of the MMTV specific DNA sequences in the host cell genome. Nucleic acid annealing and thermal denaturation of duplexes will be used to detect differences in MMTV DNA and RNA that are present in various mouse tissues. In addition, I will use mouse-hamster hybrid cells to determine the distribution of MMTV sequences in the host cell genome. The relative arrangement of MMTV sequences in the host cell genome will also be studied using endonucleases and agarose gel electrophoresis. I will thus seek to elucidate the role that viral and host cell nucleic acids play in the control of the biological expression of MMTV.